Internal combustion engine piston and manufacturing process

Abstract

[Object] An internal combustion engine piston is formed by casting with a mold and has a piston apex portion on which an intake valve recess is formed. This recess has an intake bottom wall and an intake outer side wall with an arc-shaped configuration, tapering upwards from an outer portion of the intake bottom wall located closer to an outer peripheral edge of the piston apex portion. The surface roughness of a surface of an intermediate portion in an extensional direction of the intake outer side wall is 4 μm Ra or less. Shot peening is performed on a region of the mold corresponding to an intermediate portion in an extensional direction of the intake outer side wall, and the intermediate portion of the intake outer side wall has a surface to which this mold region is transferred. Fig. 2

Description

Title of the invention: INTERNAL COMBUSTION ENGINE PISTON AND METHOD FOR MANUFACTURING THE SAME Technical field

[0001] The present invention relates to a method of manufacturing an internal combustion engine piston (a piston of an internal combustion engine) by casting, by means of a mold, i.e. relates to a method of manufacturing an internal combustion engine piston. Furthermore, the present invention relates to an internal combustion engine piston shaped by casting by means of a mold. Prior art

[0002] A piston used in an internal combustion engine, such as a heat engine, is shaped by casting. More specifically, a piston is shaped by filling a mold with molten metal, such as aluminum or an aluminum alloy, and then cooling and solidifying it.

[0003] At the time of casting, the mold surface is brought into contact with the molten metal with which the mold is filled, and is kept in contact with the casting surface of the shaped piston after the molten metal has cooled and solidified. Thus, a coating agent is applied to the mold surface, for example, for the purpose of: protecting the mold surface; ensuring a heat-retaining property to prevent coagulation of the molten metal during filling with the molten metal and ensuring fluidity (flowability) of the molten metal; and reliably releasing the shaped piston. A layer consisting of this coating agent (hereinafter referred to as the "coating layer") is formed on the mold surface. (See, for example, paragraph

[0029] of Patent Document 1) Technical Problem

[0004] However, in a case where the casting is repeated successively by means of the mold, it may happen that the coating layer is locally detached. In this case, the mold is locally retracted, and the piston is locally bulged in correspondence with this retraction. This bulging of the piston causes the appearance of a variation in the volume of the combustion chamber of the internal combustion engine, which is defined by the piston and the cylinder.

[0005] Considering the circumstances, in the method of manufacturing an internal combustion engine piston, it is desirable that the variation in the configuration of the piston is suppressed even if the casting is repeated. Furthermore, in the internal combustion engine piston, it is desirable to suppress the configuration variations. Technical solution

[0006] In order to solve the above-mentioned problem, a method according to one aspect is a method for manufacturing an internal combustion engine piston in which an internal combustion engine piston is manufactured by casting using a mold, wherein the internal combustion engine piston comprises a piston apex portion having an intake valve recess, the intake valve recess having: an intake bottom wall forming at least one bottom thereof;and an inlet outer side wall rising taperedly from an outer portion of the inlet bottom wall located closer to an outer peripheral edge of the piston apex portion, the inlet outer side wall extending over the piston apex portion so as to form a protruding arc configuration toward the outer peripheral edge of the piston apex portion, the method comprising: a roughening treatment step in which a shot peening treatment is performed on a region of the mold corresponding to an intermediate portion in an extension direction of the inlet outer side wall; and a casting step in which the piston apex portion is shaped by casting using the mold. ;

[0007] For example, the piston apex portion includes an exhaust valve recess, the exhaust valve recess has: an exhaust bottom wall forming at least a bottom thereof; and an exhaust outer side wall rising taperedly from an outer portion of the exhaust bottom wall located closer to the outer peripheral edge of the piston apex portion, the exhaust outer side wall extending over the piston apex portion so as to form an arc configuration projecting toward the outer peripheral edge of the piston apex portion, and in the roughening treatment step, a shot peening treatment is performed on a region of the mold corresponding to an intermediate portion in an extension direction of the exhaust outer side wall.

[0008] For example, the shot peening treatment of the roughening treatment step is performed on regions of the mold corresponding to the entire intermediate portions of the inlet and exhaust outer side walls.

[0009] For example, the piston apex portion has a plurality of intake valve recesses and a plurality of exhaust valve recesses, the piston apex portion has: an intake connecting wall interconnecting the intake outer side walls of adjacent intake valve recesses among the plurality of intake valve recesses; and an exhaust connecting wall interconnecting the exhaust outer side walls of adjacent exhaust valve recesses among the plurality of exhaust valve recesses, and in the roughening treatment step, the treatment by shot peening is carried out on regions of the mold corresponding to the intake and exhaust connection walls.

[0010] For example, the shot peening treatment of the roughening treatment step imparts a surface roughness of 20 pm to 50 pm Rzjis to the region of the mold on which it is performed.

[0011] In order to solve the above-mentioned problem, an internal combustion engine piston according to one aspect is an internal combustion engine piston formed by casting using a mold, the internal combustion engine piston comprising a piston apex portion having an intake valve recess, the intake valve recess having: an intake bottom wall forming at least one bottom thereof;and an inlet outer side wall rising taperedly from an outer portion of the inlet bottom wall located closer to an outer peripheral edge of the piston apex portion, the inlet outer side wall extending over the piston apex portion so as to form a protruding arc configuration toward the outer peripheral edge of the piston apex portion, and the surface roughness of a surface of an intermediate portion in an extension direction of the inlet outer side wall is 4 pm Ra or less. ;

[0012] For example, the piston apex portion includes an exhaust valve recess, the exhaust valve recess has: an exhaust bottom wall forming at least a bottom thereof; and an exhaust outer side wall rising taperedly from an outer portion of the exhaust bottom wall located closer to the outer peripheral edge of the piston apex portion, the exhaust outer side wall extending over the piston apex portion so as to form a protruding arc configuration toward the outer peripheral edge of the piston apex portion, and the surface roughness of a surface of an intermediate portion in an extension direction of the exhaust outer side wall is 4 pm Ra or less.

[0013] For example, the surface roughness of a surface in each of the entireties of the intermediate portions of the intake and exhaust outer side walls is 4 pm Ra or less.

[0014] For example, the piston apex portion has a plurality of intake valve recesses and a plurality of exhaust valve recesses, the piston apex portion has: an intake connecting wall interconnecting the intake outer side walls of adjacent intake valve recesses among the plurality of intake valve recesses; and an exhaust connecting wall interconnecting the exhaust outer side walls of adjacent exhaust valve recesses among the plurality of exhaust valves, and the surface roughness of a surface in each of the intake and exhaust connecting walls is 4 pm Ra or less. Benefits provided

[0015] In the internal combustion engine piston manufacturing method according to one aspect of the invention, variation in the configuration of the piston can be suppressed even if casting is performed repeatedly. Furthermore, in the internal combustion engine piston according to one aspect, variation in the configuration of the piston can be suppressed. Brief description of the drawings Fig.l

[0016] [fig.l] Figure 1 is a plan view schematically showing a piston apex portion of a piston according to one embodiment; Fig. 2

[0017] [fig-2] Figure 2 is a sectional view schematically showing a piston according to the embodiment and a mold corresponding to the piston apex portion, according to line AA of Fig. 1; Fig. 3

[0018] [fig.3] Figure 3 is an enlarged view of part B of Fig. 1; Fig. 4

[0019] [fig-4] Figure 4 is an enlarged view of part C of Fig. 1; Fig. 5

[0020] [fig.5] Figure 5 is a bottom view schematically showing a cor mold corresponding to a piston apex portion according to the embodiment; Fig. 6

[0021] [fig.6] Figure 6 is an enlarged view of part D of Fig. 2; Fig. 7

[0022] [fig-7] Figure 7 is an enlarged view of part E of Fig. 2; Fig. 8

[0023] [fig.8] Figure 8 is a flowchart showing a manufacturing process of piston according to the embodiment; Fig. 9

[0024] [fig.9] Figure 9 is a graph showing the surface roughness of the regions measured 1 to 4 of the pistons in an example and in a comparative example; Fig. 10

[0025] [fig.10] Figure 10A is a graph showing a relationship between a ratio of variation of the volume of a combustion chamber in a piston apex portion and the number of trials for casting the pistons by means of a mold in the example; Figure 10B is a graph showing a relationship between a volume change ratio of a combustion chamber in a piston apex portion and the number of trials for casting pistons by means of a mold in the comparative example. Description of Embodiments

[0026] A piston (an internal combustion engine piston) according to one embodiment and a manufacturing method thereof are described below. The piston of the present embodiment is formed by casting using a mold. That is, the piston is a casting. The piston of the present embodiment is to be used in an automobile engine. The piston, however, may be used in an internal combustion engine other than an automobile engine. For example, the piston may be used in the engine of a motorcycle, the engine of an outboard motor, the engine of a snow vehicle, the general-purpose engine mounted on a small lawn mower, or the like. In the manufacturing method of the present embodiment, this piston is manufactured by casting using a mold.

[0027] “Piston Overview” First, an overview of a piston 1 according to the present embodiment is described with reference to Figs. 1 to 4. As illustrated in Figs. 1 to 3, the piston 1 has a piston apex portion 2 located at one end, on the apex side thereof. An intake valve recess 3 is formed at the piston apex portion 2. The intake valve recess 3 is recessed from the piston apex portion 2 so as to avoid the intake valve (not shown).

[0028] For example, in Figs. 1 and 3, two intake valve recesses 3 corresponding to two intake valves (not shown) respectively are formed in the piston apex portion 2. However, the present invention is not limited to this structure. One intake valve recess corresponding to one intake valve may be formed in the piston apex portion, or three or more intake valve recesses corresponding to three or more intake valves respectively may be formed in the piston apex portion.

[0029] As illustrated in Fig. 3, each intake valve recess 3 has an intake bottom wall 31 forming at least a bottom thereof. Each intake valve recess 3 has an intake outer side wall 32 rising taperedly from an outer portion 31a of the intake bottom wall 31 of the intake valve recess 3 located closer to an outer peripheral edge 2a of the piston apex portion 2. As illustrated in Figs. 1 and 3, the intake outer side wall 32 extends over the piston apex portion 2 so as to form an arc-like configuration projecting toward the outer peripheral edge 2a of the piston apex portion 2.

[0030] Furthermore, the piston 1 of the present embodiment may be schematically configured as follows. As illustrated in Figs. 1 to 4, an exhaust valve recess 4 is formed at the piston apex portion 2. The exhaust valve recess 4 is recessed from the piston apex portion 2 so as to avoid an exhaust valve (not shown).

[0031] For example, in Figs. 1 and 4, two exhaust valve recesses 4 corresponding to two exhaust valves (not shown) respectively are formed in the piston apex portion 2. The present invention, however, is not limited to this structure. One exhaust valve recess corresponding to one exhaust valve may be formed in the piston apex portion, or three or more exhaust valve recesses corresponding to three or more exhaust valves respectively may be formed in the piston apex portion.

[0032] As illustrated in Fig. 4, each exhaust valve recess 4 has an exhaust bottom wall 41 forming at least one bottom thereof. Each exhaust valve recess 4 has an exhaust outer side wall 42 rising taperedly from an outer portion 41a of the exhaust bottom wall 41 of the exhaust valve recess 4 located closer to the outer peripheral edge 2a of the piston apex portion 2. As illustrated in Figs. 1 and 4, the exhaust outer side wall 42 extends over the piston apex portion 2 so as to form an arcuate configuration projecting toward the outer peripheral edge 2a of the piston apex portion 2.

[0033] As illustrated in Figs. 1, 3 and 4, in the case where the piston apex portion 2 has a plurality of intake valve recesses 3 and a plurality of exhaust valve recesses 4, the piston apex portion 2 has an intake connecting wall 5 connecting the intake outer side walls 32 of the adjacent intake valve recesses 3 within the plurality of intake valve recesses 3. Furthermore, the piston apex portion 2 is provided with an exhaust connecting wall 6 connecting the exhaust outer side walls 42 of the adjacent exhaust valve recesses 4 among the plurality of exhaust valve recesses 4.

[0034] “Piston Details” The piston 1 according to the present embodiment is described in detail with reference to Figs. 1 to 4. That is, in detail, the piston 1 may be formed as follows. The material of the piston 1 may be an aluminum alloy. However, the material of the piston is not limited to this. For example, the material of the piston may be iron, aluminum, titanium, a titanium alloy, magnesium, a magnesium alloy, or the like.

[0035] As illustrated in Fig. 1 and 2, the outer peripheral surface of the piston 1 is formed into a substantially cylindrical configuration so that the center of the outer peripheral surface of the piston 1 is substantially the piston axis 1a and the outer peripheral surface of the piston 1 surrounds the piston axis 1a. The apex of the piston 1 is located on one side in the axial direction of the piston along the piston axis 1a (indicated by the arrow P1). The bottom of the piston 1 is located on the other side in the axial direction of the piston (indicated by the arrow P2) along the piston axis 1a. As illustrated in Fig. 2, the piston apex portion 2 is arranged according to an imaginary apex surface 1b substantially orthogonal to the piston axis 1a, and is located at the apex of the piston 1. Although not specifically illustrated, the piston 1 reciprocates along the piston axis 1a inside the engine cylinder.

[0036] As illustrated in Fig. 1, the outer peripheral edge 2a of the piston apex portion 2 is formed in a substantially circular configuration. The intake and exhaust valve recesses 3 and 4 are arranged to oppose each other in a radial direction (hereinafter referred to as the recess opposing direction) of the piston apex portion 2. The plurality of intake valve recesses 3 may be arranged to respectively oppose the plurality of exhaust valve recesses 4 in the recess opposing direction.

[0037] The intake valve recess 3 is located on one side in the opposing direction of the recesses (indicated by the arrow Q1) with respect to the boundary 2b of the piston apex portion 2. The exhaust valve recess 4 is located on the other side in the opposing direction of the recesses (indicated by the arrow Q2) with respect to the boundary 2b of the piston apex portion 2. The boundary 2b is defined as a straight line, substantially orthogonal to the opposing direction of the recesses and passes substantially through the center of the piston apex portion 2.

[0038] As illustrated in Fig. 2, the intake bottom wall 31 is inclined such that the further it is from the boundary 2b in the direction of opposition of the recesses, the closer it is to the bottom of the piston 1. The intake bottom wall 31 forms the bottom of the recess for the intake valve 3, and a wall connected to the piston apex portion 2 on the side of the boundary 2b with respect to this bottom. The intake outer side wall 32 is inclined such that the further it is from the outer portion 31a of the intake bottom wall 31 toward the side of the outer peripheral edge 2a of the piston apex portion 2, the closer it is to the apex of the piston 1. The inclination of the intake outer side wall 32 relative to the imaginary apex surface 1b of the piston is greater than the inclination of the intake bottom wall 31 relative to the imaginary apex surface 1b of the piston.

[0039] The exhaust bottom wall 41 is inclined such that the further it is from the boundary 2b in the direction of opposition of the recesses, the closer it is to the bottom of the piston 1. The exhaust bottom wall 41 forms the bottom of the recess for exhaust valve 4, and a wall connected to the piston apex portion 2 on the side of the boundary 2b relative to this bottom. The outer exhaust side wall 42 is inclined such that the further it moves away from the outer portion 41a of the exhaust bottom wall 41 towards the side of the outer peripheral edge 2a of the piston apex portion 2, the closer it is to the apex of the piston 1.

[0040] The inclination of the exhaust outer side wall 42 relative to the imaginary apex surface 1b of the piston is greater than the inclination of the exhaust bottom wall 41 relative to the imaginary apex surface 1b of the piston. The inclination of the exhaust outer side wall 42 relative to the imaginary apex surface 1b of the piston may be less than the inclination of the exhaust outer side wall 32 relative to the imaginary apex surface 1b of the piston. However, the inclination of the exhaust outer side wall relative to the imaginary apex surface of the piston may be substantially equal to or greater than the inclination of the exhaust outer side wall relative to the imaginary apex surface of the piston.

[0041] As illustrated in Figs. 1 and 3, the intake connecting wall 5 extends substantially linearly over the piston apex portion 2 between the ends in the extension direction of the adjacent intake outer side walls 32. The intake connecting wall 5 is inclined such that the further it is from the boundary 2b in the opposing direction of the recesses, the closer it is to the piston apex 1. Although not clearly specifically illustrated, the inclination of the intake connecting wall 5 relative to the imaginary apex surface 1b of the piston is less than the inclination of the intake outer side wall 32 relative to the imaginary apex surface 1b of the piston.

[0042] As illustrated in Figs. 1 and 4, the exhaust connecting wall 6 extends over the piston apex portion 2 in a substantially linear manner between the ends in the extension direction of the adjacent exhaust outer side walls 42. The exhaust connecting wall 6 is inclined such that the further it is from the boundary 2b in the opposing direction of the recesses, the closer it is to the apex of the piston 1. Although not clearly specifically illustrated, the inclination of the exhaust connecting wall 6 relative to the imaginary apex surface 1b of the piston is less than the inclination of the exhaust outer side wall 42 relative to the imaginary apex surface 1b of the piston.

[0043] “Mold Details” A mold 101 used in manufacturing the piston 1 of the present embodiment is described with reference to Fig. 2 and Figs. 5 to 8. The material of the mold 101 is a material corresponding to SKD 61. However, the material of the mold is not limited thereto. For example, the material of the mold may be hardened steel. The material of the mold may also be a material of type SC, SCM, SUS or the like.

[0044] Referring to Fig. 2 and Figs. 5 to 7, the mold 101 has a piston apex region 102 corresponding to the piston apex portion 2. The piston apex region 102 has an intake valve recess region 103 corresponding to the intake valve recess 3. The intake valve recess region 103 has an intake bottom wall region 131 corresponding to the intake bottom wall 31. Furthermore, the intake valve recess region 103 has an intake outer side wall region 132 corresponding to the intake outer side wall 32.

[0045] The mold 101 has an exhaust valve recess region 104 corresponding to the exhaust valve recess 4. The exhaust valve recess region 104 has an exhaust bottom wall region 141 corresponding to the exhaust bottom wall 4L. Furthermore, the exhaust valve recess region 104 has an exhaust outer side wall region 142 corresponding to the exhaust outer side wall 42.

[0046] The mold 101 has an intake connecting wall region 105 corresponding to the intake connecting wall 5. The mold 101 has an exhaust connecting wall region 106 corresponding to the exhaust connecting wall 6.

[0047] The shot peening treatment is performed on all or part of the piston apex region 102 of this mold 101. Thus, the mold 101 has a treated region that has undergone this shot peening treatment. The surface roughness of the treated region may be approximately 20 pm to 50 pm Rzjis. However, the surface roughness of the treated region is not limited thereto. And the treated surface of the piston 1, in particular, the treated surface of the piston apex portion 2 is formed so as to have a surface roughness corresponding to the surface roughness of this treated region. In the piston 1, the surface roughness of the treated surface of the piston apex portion 2 may be approximately 4 pm Ra or less.In particular, the treated surface with a surface roughness of approximately 4 pm Ra or less can be achieved by transferring the treated region having a surface roughness of approximately 20 pm to 50 pm. However, the surface roughness of the treated surface is not limited thereto.

[0048] Referring to Figs. 6 and 7, the treated region may be an intermediate section 132a in the extension direction of the intake outer side wall region 132 corresponding to the intermediate portion 32a in the extension direction of the intake outer side wall 32. In this case, the intermediate portion 32a of the intake outer side wall 32 has a treated surface obtained by transferring the intermediate section 132a of the intake outer side wall region 132.

[0049] The treated region may be composed of the intermediate sections 132a and 142a in the extension direction of the intake and exhaust outer sidewall regions 132 and 142 which correspond respectively to the intermediate portions 32a and 42a in the extension direction of the intake and exhaust outer sidewalls 32 and 42. In this case, the intermediate portions 32a and 42a of the intake and exhaust outer sidewalls 32 and 42 have treated surfaces obtained by transferring the intermediate sections 132a and 142a of the intake and exhaust outer sidewall regions 132 and 142.

[0050] The treated region may be the set of intake and exhaust outer sidewall regions 132 and 142. In this case, the set of intake and exhaust outer sidewalls 32 and 42 has the treated surface obtained by transferring the set of intake and exhaust outer sidewall regions 132 and 142.

[0051] The treated region may be all of the intake and exhaust outer sidewall regions 132 and 142, and the intake and exhaust connecting wall regions 105 and 106. In this case, all of the intake and exhaust outer sidewalls 32 and 42 and the intake and exhaust connecting walls 5 and 6 have treated surfaces obtained by transferring all of the intake and exhaust outer sidewall regions 132 and 142, and the intake and exhaust connecting wall regions 105 and 106.

[0052] The treated region may be the regions for recesses for intake and exhaust valves 103 and 104, as well as the regions for intake and exhaust connecting walls 105 and 106. In this case, the recesses for intake and exhaust valves 3 and 4, and the intake and exhaust connecting walls 5 and 6 have treated surfaces obtained by transferring the regions for recesses for intake and exhaust valves 103 and 104 and the regions for intake and exhaust connecting walls 105 and 106.

[0053] The treated region may be the piston apex region 102. In this case, the piston apex portion 2 has a treated surface obtained by transferring the piston apex region 102.

[0054] “Piston manufacturing method” Referring to Fig. 8, a method for manufacturing the piston 1 according to the present embodiment is now described. That is, the method for manufacturing the piston 1 by means of the mold 101 is as follows. Shot peening treatment is performed on the treated region of the mold 101 (roughening treatment step S1). The piston apex portion 2 is shaped by casting aluminum alloy by means of the mold 101 (casting step S2).

[0055] More specifically, the casting step S2 proceeds as follows. The molten aluminum alloy is poured into the mold 101. The mold 101 is filled with the metal melted so that the molten metal is poured throughout the piston apex region 102. The mold 101 filled with the molten metal is allowed to stand for a fixed period of time until the molten metal is cooled and solidified so as to determine the configuration of the piston apex portion 2. After this, the shaped piston 1 is extracted from the mold 101.

[0056] Furthermore, after the roughening treatment step S1 and before the casting step S2, a surface treatment for suppressing wear may be performed on the treated region of the mold 101 (Surface Treatment Step). For example, the surface treatment may be a molten salt carbonization treatment, CVD (chemical vapor deposition), PVD (physical vapor deposition), DLC (diamond-like carbon) coating, or the like. By this surface treatment, it is possible to suppress a material base on the material of the piston 1 such as an aluminum alloy from being transferred to a part of the mold 101 that is difficult to release. Accordingly, it is possible to reduce the frequency of cleaning the mold 101, reprocessing, or the like.

[0057] In the manufacturing method of the piston 1 of the present embodiment described above, a shot peening treatment is performed on all or part of the piston apex region 102 of the mold 101, and the mold 101 has a treated region having undergone a shot peening treatment. Furthermore, in the piston 1 of the present embodiment, the surface of the piston apex portion 2 is a treated surface obtained by transferring the piston apex region 102 having undergone a partial or complete shot peening treatment. Thus, even in the case where casting is carried out repeatedly in succession by means of the mold 101, there is no fear of the coating layer of the mold flaking off in the treated region of the mold 101 as is the case in the prior art.Thus, it is possible to prevent deformation of the mold 101, resulting in the possibility of eliminating variation in the configuration of the manufactured piston 1.

[0058] In this case, the present inventor has acquired the following new knowledge regarding the principle according to which the coating layer peels off in the case where the coating agent is applied to the mold surface as in the prior art. Generally, a recessed portion is formed at the piston apex portion, and a protruding portion corresponding to the recessed portion of the piston apex portion is formed on the mold surface. In particular, a side wall rising from the bottom wall of the recessed portion at the piston apex portion is inclined so as to provide a draft or the like, and the side wall of the protruding mold portion corresponding to this side wall is also inclined. As the inclination of the surface of the side wall of the protruding mold portion increases, It becomes difficult to apply the coating agent to the side wall of the mold protrusion. Therefore, the coating layer on the side wall of the mold protrusion may be thinner than the coating layer on the other part of the mold. In addition, it becomes difficult to achieve a fixed coating layer thickness, and the coating layer thickness may be uneven.

[0059] In this case, at the side wall of the protruding portion, the thermal insulation performance of the coating layer deteriorates, and the temperature of the side wall of the protruding portion is more easily increased than the temperature of the other portion. In particular, the contact area with the molten metal at the protruding portion of the mold is larger than the contact area with the molten metal at the other portion of the mold, so that the temperature of the protruding portion is likely to become higher than the temperature of the other portion.In the case where casting is repeatedly performed successively by means of this mold, the adhesion property between the side wall of the protruding part of the mold and the coating layer deteriorates, and, due to the deterioration of the adhesion property, the coating layer located on the side wall of the protruding part of the mold becomes prone to peeling. Furthermore, in the thin coating layer located on the side wall of the protruding part of the mold, in the coating layer of uneven thickness, or the like, an undercut may be generated and, due to this undercut, a resistance peeling the coating layer upon release of the plunger may apply.

[0060] The inclinations of the piston 1 of the outer configuration described above, and in particular the inclinations of the piston apex portion 2, are now compared with each other. The inclination of the intake and exhaust outer side walls 32 and 42 in the piston apex portion 2, and the inclination of the intake and exhaust connecting walls 5 and 6 are greater than the inclination of the other parts in the piston apex portion 2. The inclination of the intake and exhaust outer side walls 32 and 42 is greater than the inclination of the other parts in the piston apex portion 2. The inclination of the intermediate parts 32a and 42a in the extension direction of the intake and exhaust outer side walls 32 and 42 is greater than the inclination of the other parts in the piston apex portion 2.The inclination of the intermediate portion 32a in the extension direction of the intake and exhaust outer side wall 32 is greater than the inclination of the other portions in the piston apex portion 2. .

[0061] In the case where, on the basis of either of these inclination comparisons, the large inclination portion of the piston apex portion 2 is defined as the portion inclined more significantly than the other portions of the piston apex portion 2, when the coating layer is applied to the piston apex portion 2, the coating layer at the large slope portion formed by the coating agent is more prone to flaking than the coating layer on other portions.

[0062] In contrast, in the manufacturing method according to the present embodiment, instead of applying the coating agent to the large inclination portion of the piston apex portion 2, a shot peening treatment is performed thereon. Furthermore, in the piston 1 according to the present embodiment, the surface of the large inclination portion of the piston apex portion 2 is the treated surface obtained by transferring the piston apex region 102 having undergone a shot peening treatment. Thus, even in the case where casting is performed repeatedly in succession by means of the mold, it is possible to prevent a change in the configuration of the mold attributable to the peeling of the coating layer of the mold as is the case in the prior art, thereby suppressing a variation in the configuration of the piston 1 attributable to a change in the configuration of the mold.

[0063] Furthermore, generally, in the case where no coating agent is applied to the mold region, a thermally insulating layer is formed due to the protrusions and recesses attributable to the roughness of the mold region. Thus, when the surface roughness of the mold region is too low, this thermally insulating layer is not formed to a sufficient degree, so that a flowability defect may be generated during casting. In contrast, in the manufacturing method according to the present embodiment, the surface roughness of the treated region of the mold 101 is set to about 20 qm Rzjis or more, so that it is possible to form a thermally insulating layer to a sufficient degree, with the result that a flowability defect may be prevented from being generated during casting.

[0064] Furthermore, generally speaking, in the case where casting is repeatedly performed successively by means of the mold, the surface of the mold region deteriorates, so that the surface roughness of the casting surface of the piston may be deteriorated. On the other hand, in the manufacturing method of the present embodiment, the surface roughness of the treated mold region 101 is set to a value of about 50 qm Rzjis or less, so that even in the case where casting is repeatedly performed by means of the mold 101, it is possible to prevent the surface roughness of the treated mold region 101 from being deteriorated so as to deteriorate the surface roughness of the casting surface of the piston 1.

[0065] Furthermore, in the piston 1 of the present embodiment, the surface roughness of the piston 1, in particular, the surface roughness of the treated surface of the piston apex portion 2 may be approximately 4 qm Ra or less. Therefore, even in the case where casting is repeatedly performed by means of the mold 101 for the piston 1, damage to the mold can be prevented so that it is possible to suppress the variation in the configuration of the piston 1 so that it is limited to a small range. In particular, in the case where a coating layer is formed on a surface of a mold as in the prior art, it is difficult to suppress the surface roughness of a surface of a piston cast by means of this mold so that it reaches approximately 4 pm Ra or less. In contrast, in the present embodiment, the surface roughness of the piston 2, in particular, the surface roughness of the treated surface of the piston apex portion 4 can be approximately 4 pm Ra or less. Accordingly, it is possible to suppress the variation in the configuration of the piston 1 so that it is within a smaller range than in the prior art.

[0066] It is noted that the surface roughness of the piston 1, in particular the surface roughness of the treated surface of the piston apex portion 2 can be set to be equal to or greater than a limit value, the limit value obtained by transferring a surface of a mold on which a shot peening treatment has been performed. Furthermore, the surface roughness of the piston 1, in particular the surface roughness of the treated surface of the piston apex portion 2 can be set to be greater than approximately 0 (zero).

[0067] The present invention is not restricted to the embodiment described above but allows variations and modifications based on the technical idea thereof.

[0068] “Example” An example is now described. In this example, the pistons 1 according to the above embodiment were manufactured by means of the mold 101 according to the embodiment. Specifically, the shot peening treatment was performed on each of the two regions for intake valve recesses 103 and the two regions for exhaust valve recesses 104 in the piston apex region 102 of the mold 101, for the purpose of maintaining the surface roughness of the treated surface in each of the two intake valve recesses 3 and the two exhaust valve recesses 4 at approximately 4 μm Ra or less in each of the piston apex portions 2 of the pistons 1 even in the case where 4500 casting tests of the pistons 1 by means of the mold 101 were carried out. 4500 casting tests using this mold were carried out so that 4500 copies of piston 1 were manufactured.

[0069] In the example, the surface roughness of the treated surface in each of the two intake valve recesses 3 and the two exhaust valve recesses 4 in the piston apex portion 2 of the piston 1 which was obtained in the 4500th casting test was measured. It is noted that, hereinafter, one of the two valve recesses of intake valve 3 was called "Measurement Region 1", the other of the two recesses for intake valves 3 was called "Measurement Region 2", one of the two recesses for exhaust valves 4 was called "Measurement Region 3", and the other of the two recesses for exhaust valves 4 was called "Measurement Region". As for the surface roughness measurement conditions, the measurements were carried out based on "JIS B 0633:2001". The following other parameters were used for surface roughness measurements: a roundness (R) of a distal end of a detection needle used in the measurements was chosen as 2 pm, a feed rate of the detection needle was chosen as 0.5 mm / sec, a cut-off value Xc was chosen as 0.8 mm, and a measured length was chosen as 2.5 mm.

[0070] Furthermore, in the example, a volume of a combustion chamber in each piston apex portion 2 of the pistons 1 which were obtained respectively by the 500th, 1000th, 1500th, 2000th and 2500th tests among the castings, was measured by a laser measuring machine. A ratio of each of these volumes to a design value of the combustion chamber volume was calculated (hereinafter referred to as "combustion chamber volume change ratio" when necessary).

[0071] “Comparative example” A comparative example is now described. In the comparative example, pistons were manufactured using a prior art mold. Specifically, a coating layer was formed on each of two intake valve recess regions and two exhaust valve recess regions in a piston apex region of the mold, as in the prior art. 4500 casting trials using this mold were performed so that 4500 examples of pistons were manufactured.

[0072] In the comparative example, the surface roughness of the treated surface in each of the two indentations for intake valves and the two indentations for exhaust valves in the piston apex portion of the piston which was obtained in the 100th casting test was measured. It is noted that in the comparative example, the names "Measurement region 1" to "Measurement region 4" were used in the same manner as the above-mentioned example. The conditions of the surface roughness measurements of the comparative example were the same as the conditions of the surface roughness measurements of the example.

[0073] Furthermore, in the comparative example, a volume of a combustion chamber in each piston apex portion of the pistons which were obtained respectively by the 500th, 1000th, 1500th, 2000th and 2500th tests among the castings, was measured by a laser measuring machine. A ratio of each of these volumes to a design value of the volume of the combustion chamber, i.e., a variation ratio of the volume of the combustion chamber, was calculated.

[0074] The surface roughness measurement results of the example and the comparative example were obtained as shown in the graph of Fig. 9. It is noted that in the graph of Fig. 9, a vertical axis Ra shows a surface roughness measurement value (pm), and M1, M2, M3 and M4 on a horizontal axis show Measurement Regions 1, 2, 3 and 4, respectively.

[0075] As illustrated respectively by means of hatched bars in the graph of Fig. 9, the measured values ​​of surface roughness in Measurement Regions 1, 2, 3 and 4 of the example were 2.452 pm, 2.386 pm Ra, 3.503 pm Ra and 3.137 pm Ra, respectively. In contrast, as illustrated respectively by means of white bars in the graph of Fig. 9, the measured values ​​of surface roughness in Measurement Regions 1, 2, 3 and 4 of the comparative example were 5.989 pm Ra, 4.620 pm Ra, 5.567 pm Ra and 4.543 pm Ra, respectively. Therefore, it could be confirmed that it was difficult to maintain the surface roughness of the Measurement Regions 1 to 4 at 4 pm Ra or less in the comparative example. In contrast, it could be confirmed that it was effective to use the mold 101 on which shot peening was performed as in the example, to maintain the surface roughness of the piston 1 at 4 pm Ra or less.

[0076] Next, a relationship between the combustion chamber volume change ratio and the number of casting trials of the pistons by means of the mold was obtained in each of the example and the comparative example, as illustrated in the graphs of Figs. 10A and 10B. Note that in each of Figs. 10A and 10B, a vertical axis Y indicates the combustion chamber volume change ratio, and a horizontal axis X indicates the number of casting trials by means of the mold.

[0077] As shown by a solid line L1 in the graph of Fig. 10B, in the example, even in the case where 2500 casting trials using the mold 101 were performed, the variation ratios of the combustion chamber volumes based on the pistons 1 obtained by these castings were in a range of 0.995 to 1.005. In contrast, as shown by a solid line L2 in the graph of Fig. 10B, in the comparative example, in the case where 2500 casting trials using the mold were performed, the variation ratios of the combustion chamber volumes based on the pistons obtained by these castings were in a range of 0.990 to 1.010.Therefore, it could be confirmed that the variation of a configuration of a piston cast by means of a mold on which shot peening was performed was less than the variation of a configuration of a piston cast by means of a mold on which a coating layer was formed as in the prior art. List of reference signs

[0078] 1 ... Piston 2... Piston apex part 2a ... Outer peripheral edge 3 ... Removal for intake valve 31 ... Intake back wall 31a... Outer part 32 ... Intake outer side wall 32a... Intermediate part 4 ... Removal for intake valve 41 ... Exhaust bottom wall 41a... Outer part 42 ... Exhaust outer side wall 42a... Intermediate part 5 ... Intake connecting wall 6 ... Exhaust connecting wall 101 ... Mold IF ... Roughening treatment step S2 ... Pouring stage List of cited documents Patent documents

[0079] - [Patent Document 1] JP 2018-178848 A.

Claims

Claims

1. A method of manufacturing an internal combustion engine piston (1) in which an internal combustion engine piston is manufactured by casting using a mold (101), wherein the internal combustion engine piston comprises a piston apex portion (2) having an intake valve recess (3), the intake valve recess having: an intake bottom wall (31) forming at least one bottom thereof;and an inlet outer side wall (32) rising taperedly from an outer portion (31a) of the inlet bottom wall (31) located closer to an outer peripheral edge (2a) of the piston apex portion (2), the inlet outer side wall (32) extending over the piston apex portion (2) so as to form an arc configuration projecting toward the outer peripheral edge (2a) of the piston apex portion (2), the method comprising: a roughening treatment step (S1) in which a shot peening treatment is performed on a region of the mold (101) corresponding to an intermediate portion (32a) in an extension direction of the inlet outer side wall (32); and a casting step (S2) in which the piston apex portion (2) is shaped by casting using the mold (101).;

2. A method of manufacturing an internal combustion engine piston (1) according to claim 1, wherein the piston apex portion (2) comprises an exhaust valve recess (4), the exhaust valve recess having: an exhaust bottom wall (41) forming at least one bottom thereof;and an exhaust outer side wall (42) rising taperedly from an outer portion of the exhaust bottom wall (41) located closer to the outer peripheral edge (2a) of the piston apex portion (2), the exhaust outer side wall (42) extending over the piston apex portion (2) so as to form an arc configuration projecting toward the outer peripheral edge (2a) of the piston apex portion (2), and in the roughening treatment step (SI), a shot peening treatment is performed on a region of the mold (101) corresponding to an intermediate portion (42a) in an extension direction of the exhaust outer side wall (42).;

3. Method for manufacturing internal combustion engine piston (1) according to claim 2, wherein the shot peening treatment of the roughening treatment step (SI) is carried out on regions of the mold (101) corresponding to all of the intermediate portions (32a, 42a) of the outer intake (32) and exhaust (42) side walls.

4. A method of manufacturing an internal combustion engine piston (1) according to claim 2 or 3, wherein the piston apex portion (2) has a plurality of intake valve recesses (3) and a plurality of exhaust valve recesses (4), the piston apex portion (2) has: an intake connecting wall (5) connecting together the intake outer side walls (32) of adjacent intake valve recesses (3) among the plurality of intake valve recesses (3); and an exhaust connecting wall (6) connecting together the exhaust outer side walls (42) of adjacent exhaust valve recesses (4) among the plurality of exhaust valve recesses (4), and in the roughening treatment step (SI), the shot peening treatment is performed on regions of the mold (101) corresponding to the intake (5) and exhaust (6) connecting walls.

5. A method of manufacturing an internal combustion engine piston (1) according to one of claims 1 to 4, wherein the shot peening treatment of the roughening treatment step (SI) imparts a surface roughness of 20 qm to 50 qm Rzjis to the region of the mold (101) on which it is performed.

6. An internal combustion engine piston (1) formed by casting using a mold (101), the internal combustion engine piston comprising a piston apex portion (2) having an intake valve recess (3), wherein the intake valve recess has: an intake bottom wall (31) forming at least one bottom thereof;and an inlet outer side wall (32) rising taperedly from an outer portion (31a) of the inlet bottom wall (31) located closer to an outer peripheral edge (2a) of the piston apex portion (2), the inlet outer side wall (32) extending over the piston apex portion (2) so as to form an arc-like configuration projecting toward the outer peripheral edge (2a) of the piston apex portion (2), and the surface roughness of a surface of an intermediate portion (32a) in an extension direction of the inlet outer side wall (32) is 4 μm; Ra or less.

7. An internal combustion engine piston (1) according to claim 6, wherein the piston apex portion (2) comprises an exhaust valve recess (4), the exhaust valve recess having: an exhaust bottom wall (41) forming at least one bottom thereof; and an exhaust outer side wall (42) rising taperedly from an outer portion (41a) of the exhaust bottom wall (41) located closer to the outer peripheral edge (41a) of the piston apex portion (2), the exhaust outer side wall (42) extending over the piston apex portion (2) so as to form an arc configuration projecting toward the outer peripheral edge (2) of the piston apex portion (2), and the surface roughness of a surface of an intermediate portion (42a) in an extension direction of the exhaust outer side wall (42) is 4 pm Ra or less.

8. An internal combustion engine piston (1) according to claim 6 or 7, wherein the surface roughness of a surface in each of the intermediate portions (32a, 42a) of the intake (32) and exhaust (42) outer side walls is 4 pm Ra or less.

9. An internal combustion engine piston (1) according to one of claims 6 to 8, wherein the piston apex portion (2) has a plurality of intake valve recesses (3) and a plurality of exhaust valve recesses (4), the piston apex portion (2) has: an intake connecting wall (5) connecting together the intake outer side walls (32) of adjacent intake valve recesses (3) among the plurality of intake valve recesses (3); and an exhaust connecting wall (6) connecting together the exhaust outer side walls (42) of adjacent exhaust valve recesses (4) among the plurality of exhaust valve recesses (4), and the surface roughness of a surface in each of the intake (5) and exhaust (6) connecting walls is 4 pm Ra or less.

Images